U.S. Pat. No. 3,599,436 describes the method and apparatus for joining a subsea pipeline to a riser wherein a submersible work chamber is placed over the adjacent ends of the submerged pipeline and the riser. The seawater is then evacuated from the chamber and the pressure is maintained to permit divers to weld the ends together. This technique is limited to relatively shallow depths of approximately 250 to 300 feet. Another method and apparatus for joining a subsea pipeline to a riser is shown in U.S. Pat. No. 3,967,462 in which a submersible work chamber is attached to an offshore platform. A winch on the platform is attached to the cable passing through the riser and chamber and is used to pull an end of the pipeline into the chamber. Personnel utilizing a capsule then enter the chamber for completing and joining the pipeline and riser. One problem encountered is determining the size and shape of the small section of pipe, commonly referred to as a pup, and joining the pipeline to the riser.
One technique used in the past is to take a straight section of pipe and using an iterative method, miter it until there was a reasonably good fit and then weld the miter pup in position. This technique is less than ideal because the ends of a mitered pipe are eliptical and not circular and except for very small angles cannot be easily welded to the round pipes.
Another previously used technique is to weld together pieces of pipe and then transport those pieces to a surface vessel where the measurements are taken so that a fitted section of pipe can be cut.